The present invention relates to a decentralized control system defining a network by configuring devices in a decentralized environment and connecting devices by transmission paths composed of pairs of transmission lines between devices, and controlling control objects with devices configured in a decentralized environment.
In the field of Process Automation, the so-called field device includes the sensors used for detecting the physical values of the plant process, such as pressure, temperature and flow rate, and for converting the detected values to the electric signal and transmitting them through the transmission path to the upper-level controller (upper-level apparatus), and the actuators such as valves used for receiving the control signal transmitted from the upper-level controller and controlling the plant process such as flow rate.
In the field of Factory Automation, the field device includes the motors for driving the machines and the photo sensor for detecting the position for the upper-level controller controlling the conveying system.
In the device connection configuration of the conventional system, the upper-level controller and the field device is directly connected to each other, and the upper-level controller controls the individual field device. Therefore, the field device can not be controlled without the upper-level controller.
In the field of Process Automation, the standardized transmission line using the analog current signal between 4 and 20 mA is commonly used for the connection line between the field device and the upper-level controller.
In recent years, due to the advances in semiconductor integrated circuit technology, the field device in which microprocessors are embedded is developed and used in practical use. With this microprocessor-based field device, plural field devices are connected on the identical transmission path in the form of multi drop configuration, and the network is configured with the bi-directional digital signal communication, and not only the transmission of the instrument and control information but also the range setup and self diagnosis of the field device can be controlled in the remote environment.
Field Bus, LonWork and DeviceNet are major products in the field network technology, and the decentralized network system is extensively used for the field device level architecture in various field of control systems.
A representative example of the configuration of Field Bus system is described by referring to FIG. 3 as one of the field network systems.
FIG. 13 shows an example of the apparatus configuration in which plural field devices and the upper-level controller are connected through the transmission path in a tree topology, and is a typical instrument and control system using Field Bus. In this system, the field device is operated with the electric power supplied through the transmission path by the external power supply embedded inside the upper-level apparatus (controller), and the field device communicates digital signals with the controller in the bi-directional communication mode for transmitting the detected physical values and receiving the control value. The upper-level communication apparatus (communicator) is connected between the field device and the upper-level apparatus, and communicates digital signals with the field device in the bi-directional communication mode. The terminator (not shown) is composed of resistors and condensers connected in series and connected between the both lines of the transmission path.
In the field network system described above, as plural devices are connected on the transmission path, in case that any one of the field devices has an abnormal state, it is required to transfer the information reporting the abnormal state promptly to the related devices and controllers in order to keep the whole network system from shutdown, and furthermore, it is expected that the field device which recognizes the information related to the abnormal state control the plant process so as to transit to the safer mode.
In order to attain the above requirement, the conventional system uses the following communication method and network control method.
At first, by referring to Japanese Patent Application Laid-Open No. 4-332099 (1992), a typical method for emergency communication operation when the abnormal state generates in the field bus system is described. The configuration of the communication method disclosed in Japanese Patent Application Laid-Open No. 4-332099 (1992) is shown in FIG. 14.
FIG. 14 shows an apparatus configuration in which plural field devices and a single upper-level controller are connected in a bus topology, representing a part of the basic field bus system. In FIG. 14, the field devices 1a, 1b and in are operated with the electric power supplied through the transmission path 5 by the external power supply embedded inside the upper-level controller, and the field device communicates digital signals with the upper-level controller 3 in the bi-directional communication mode for transmitting the detected physical values and receiving the control value.
Next, by referring to FIG. 15, the basic communication signal transmission process is described.
FIG. 5(a) shows the command issued by the upper-level controller; FIG. 5(b) shows the response of the field device 1a; FIG. 15(c) shows the response of the field device 1b; FIG. 5(n) shows the response of the field device in; and FIG. 5(z) shows the scan interval of the upper-level controller. The upper-level controller submits the command signal CMDa onto the transmission path 5 for calling the field device 1a, the field device 1a detects the command signal CMDa and sends back the measurement values such as its status and measured temperature as the response RESa to the upper-level controller. Thus, the upper-level controller collects the data measured at the field device 1a. Next, the upper-level controller submits the command signal CMDb onto the transmission path 5 for calling the field device 1b, the field device 1b detects the command signal CMDb and sends back the measurement values such as its status and measured temperature as the response RESb to the upper-level controller. So far, by repeating also the above operation similarly for the rest of the field devices, the upper-level controller collects the data from the field devices 1a to 1n sequentially.
In the communication method as described above, there is such a problem that, in case that the field device has an abnormal state, the abnormal state can not be reported by interrupting the communication interval. In the communication method disclosed in Japanese Patent Application Laid-Open No. 4-332099 (1992), for the method of transmitting the abnormal state of the device promptly under the predefined scheduled communication condition, the following scheme is proposed; the signal formed with the communication frequency different from that used for the normal signal communication by the field device is transmitted as the signal for interrupting the predefine scheduled communication operation, and after the upper-level controller detects this interruption signal and suspends the communication operation, the ID code of the field device to which the shutdown or suspend request is issued is transmitted from this field device to the upper-level controller, and then, the upper-level controller identifies the field device having an abnormal state.
Next, an example of controlling the network under emergency operation is described.
As plural field device are connected to a single transmission path in the decentralized field network system such as Field Bus System, in case that a single field bus happens to run away out of control due to any reason, it is required to reset the whole bus system in order to resolve this abnormal status. However, in case that the whole bus system can not continue to communicate signals due to the runaway field device, the whole bus system can not be reset under on-line mode but it is required to reset the whole bus system by shutting down the electric supply to the individual field device in the manual operation.
In case that the transmitting circuit of the field device has an abnormal state and the communication signal can not be transmitted from the field device, there is such a problem that the field device can not restore its abnormal state by itself and report its abnormal state to another device.
In order to resolve the above described problems, the following method is proposed in Japanese Patent Application Laid-Open No.9-130312 (1997); in case that the field device itself fails to detect its abnormal state or one field device fails to detect an abnormal state of the other field device and that the signal communication is disturbed due to this kind of detection failure, the electric current consumption of the field device is forced to be increased temporarily so that the voltage between the transmission lines at the field side may be the minimum operating voltage or smaller, and then, the individual field devices are forced to be reset in order to restore the abnormal state.
However, in the conventional communication method under the emergency operation, in case that the number of the connected field devices is large, though the information related to the abnormal state of the field device can be promptly transmitted to the upper-level controller, the following problems can not be resolved.
1. As the case that the upper-level controller has an abnormal state is not assumed, the abnormal state of the overall system can not be rescued. PA1 2. The operation for resolving the abnormal state with interacting with the upper-level controller is so complex and the prompt rescue operation is difficult. PA1 3. As the signal communication between the field devices can not be realized, the abnormal state of the overall system can not be resolved quickly by exchanging the information related to the abnormal state directly between the field devices. PA1 4. In case that the communication circuit in the field device has an abnormal state, the field device having an abnormal state can not be identified. PA1 the field device has a normal communication means for communicating with other devices in a normal operation mode, and an emergency communication means for communicating with other devices in an emergent operation mode, in which the normal communication means and the emergency communication means use their own communication signals and the emergency communication means operates in an asynchronous communication mode. PA1 a shutdown control means is formed on the network for detecting the abnormal state of the individual device and, if necessary, supplying the shutdown command to the shutdown target control device for which the shutdown operation is required. PA1 an abnormality detection means for detecting an abnormal state of the individual device on the network; and PA1 a shutdown control circuit for sending a shutdown command through the control signal line.
In the conventional method for the network control under emergency situation, though the abnormal state of the field device can be resolved by resetting the whole bus system, in considering not only the abnormal state of the field device itself but also the abnormal state of the whole plant process, the continuity of the control operation of the field devices is not considered so that the system may be shutdown and the plant process may be moved to the safer mode. Therefore, the decentralized control system using the network such as Field Bus can not be applied to the system components and part which requires the highly reliable architecture for shutdown control, and hence, the shutdown control loop should be separately configured for the system components requiring the highly reliable architecture.
In many conventional field network systems mainly using digital signals for the communication operation, the reliability for noise resistance should be considered more intensively, and there is such a problem that the normal control operation for the field device scheduled in a constant time interval can not be performed if the communication error happens so often.